Two particles of equal masses have velocities $\vec{v}_{1}=2 \hat{i} \mathrm{~m} / \mathrm{s}$ and $\vec{v}_{2}=2 \hat{j} \mathrm{~m} / \mathrm{s}$. The first particle has an acceleration $\vec{a}_{1}=(3 \hat{i}+3 \hat{j}) \mathrm{m} / \mathrm{s}^{2}$, while the acceleration of the other
particle is zero. The centre of mass of the two particles moves in a
(a) circle
(b) parabola
(c) straight line
(d) ellipse